Focused ultrasound on both sides of the brain for Parkinson's brings new questions about safety and balance

Focused ultrasound has generated huge excitement in the Parkinson's community over recent years. Because it does not require incisions or implanted hardware, many people view it as an appealing alternative to traditional brain surgery. However, a newly published series of letters from leading neurosurgeons and neurologists in The Lancet Neurology urges caution when using this technology on both sides of the brain. The discussion centres on a specific procedure called focused ultrasound pallidothalamic tractotomy. Using magnetic resonance imaging for guidance, clinicians direct sound waves at a tiny bundle of nerve fibres deep inside the brain to create a permanent microscopic lesion, which is a tiny area of heat treated tissue. This interrupts the faulty signals that cause motor complications like tremors, stiffness, and involuntary movements when medication wears off. Initially, this procedure was only performed on one side of the brain to help symptoms on the opposite side of the body. In July 2025, the US Food and Drug Administration approved a staged bilateral version of the treatment. This means a person has one side of their brain treated first, and after a recovery period, returns to have the exact same spot treated on the other side. Now that the full clinical trial data behind that approval has been published by researcher Arif Dalvi and his colleagues, three international teams of specialists from the UK, the US, France, Sweden, and Japan have stepped forward to share their analysis. Their core message is simple: while treating one side of the brain works well and is relatively safe, treating the second side offers only small additional gains while bringing a much higher risk of permanent side effects. A modest boost in symptom relief, but a jump in side effects When looking at the numbers from the clinical trial, the researchers pointed out a stark contrast between the first procedure and the second. Treating the first side of the brain provided meaningful, lasting relief for motor symptoms. In fact, five people in the trial experienced such good improvements across their whole body from the single treatment alone that they decided they did not even need the second procedure. For the 40 participants who did go ahead with the second treatment on the opposite side of the brain, the extra benefit was relatively modest. Their motor scores when off medication improved by an additional 19% over the plateau they had already reached after the first procedure. However, that 19% gain came with a notable catch. One year after having the second side treated, 10 out of the 40 people, which is a full quarter of the group, were living with moderate or severe side effects that had not gone away. These persistent difficulties mainly affected their speech, walking, and balance. One participant lost the ability to speak clearly altogether, a severe condition known as anarthria. What troubled the reviewing doctors most was that the clinical trial had been very careful about who was allowed to have the second procedure. Anyone who experienced lingering moderate or severe neurological side effects after the first treatment was excluded from getting the second side done. Even in this carefully selected group of people who sailed through the first procedure, a quarter still developed long term problems after the second. Why treating both sides of the brain is so tricky To understand why this happens, it helps to look back at the history of Parkinson's surgery. Before deep brain stimulation became the standard surgical option, surgeons frequently performed physical lesioning procedures using heated probes. Back then, doctors learned a hard lesson: the brain can usually compensate well when a tiny circuit is interrupted on one side, but if you make the exact same permanent change in the mirror image spot on the other side, the brain's central control for speaking, swallowing, and balancing can become overwhelmed. As neurosurgeons Sami Barrit, Romain Carron, and Marwan Hariz noted in their letter, applying a modern, incisionless tool to the brain does not change how the brain's anatomy reacts to having both sides altered. Because focused ultrasound creates a permanent change in the tissue, any resulting problems with balance or speech cannot be undone. Japanese neurosurgeons Shiro Horisawa and Takaomi Taira highlighted that balance and walking difficulties are particularly serious for people with Parkinson's, as they increase the risk of falls and long term loss of independence. They suggested that if a second treatment is truly needed, clinicians should explore targeting two different spots on the brain's movement pathway rather than hitting the exact same spot twice. In their own early research, varying the target on the second side helped preserve people's balance and walking. A fixed treatment for a changing condition A third group of specialists, including neurologists and neurosurgeons from the University of Florida, raised another vital point for people with Parkinson's to consider: how a treatment ages over time. Parkinson's is a dynamic condition that changes and progresses over the years, meaning a person's needs today might look very different five years from now. A focused ultrasound lesion is fixed and unchanging. By comparison, while deep brain stimulation does require surgery to implant fine wires into both sides of the brain, the electrical stimulation it delivers is adjustable. If someone with deep brain stimulation develops slurred speech or balance trouble, a clinician can adjust the settings on their device to dial back the side effects while keeping symptoms under control. With newer adaptive systems, the device can even adjust itself in real time as needs fluctuate. With focused ultrasound, once the tissue is treated, there are no settings to adjust. What this means for people considering focused ultrasound None of the experts writing in The Lancet Neurology are suggesting that focused ultrasound should be abandoned. On the contrary, they emphasise that these concerns should not take away from the very encouraging results of treating one side of the brain. For someone struggling with severe tremors or stiffness primarily on one side of their body, unilateral focused ultrasound remains a compelling, incisionless option that can bring substantial relief. Where extreme care is needed is the decision to go back for a second treatment on the other side. If you or a loved one are exploring focused ultrasound, these letters highlight a few grounded questions to discuss with your neurologist and neurosurgeon. First, ask whether you can achieve enough relief by treating just one side, as getting the primary side treated may improve daily life enough that a second procedure is not necessary. Second, discuss how a second treatment might affect your speech and balance, weighing the potential for a modest improvement in stiffness or tremor against a one in four risk of long term challenges with walking, balance, or speaking. Finally, consider whether an adjustable therapy would suit your long term needs better. If treating both sides of the body is essential, discussing how an adjustable therapy like deep brain stimulation compares to a permanent procedure can help ensure your treatment can adapt as your condition evolves over time. New medical technology is at its best when paired with open, honest conversations about both its benefits and its limits. Understanding the major difference between treating one side of the brain and treating both empowers people with Parkinson's to make the safest, most informed choices for their future.